Simple infinite terrain concept BGE, HUGE drop in frame rate on spawned land
I actually solved my own problem. I had never used the near sensor to spawn an object before so I didn't realise it will constantly spawn a new object every time you move within the sensor's range. So, I simply added an "end object" actuator to the empty below the "add object(landmass)" actuator. This ensures a landmass will only ever spawn once and not spawn again if the player returns to the area.

If anyone is wondering what method I used for the spawn setup: I have each square landmass in a second layer with 3 empties on every side except for one (the one "player"[property] just came from) with a distance 1.5x the landmass width from the center of said landmass. I then simply have each empty rotate the correct 90 degree interval to spawn the new landmass in the correct orientation.